Fabrication of conductive lines interconnecting conductive gates in nonvolatile memories, and non-volatile memory structures

ABSTRACT

In a nonvolatile memory, the select gates ( 144 S) are formed from one conductive layer (e.g. polysilicon or polyside), and the wordlines ( 144 ) interconnecting the select gates are made from a different conductive layer (e.g. metal). The wordlines overlie an interlevel dielectric ( 310 ) formed over control gates ( 134 ). The dielectric thickness can be controlled to reduce the capacitance between the wordlines and the control gates. In some embodiments, the floating gates ( 120 ) are fabricated in a self-aligned manner using an isotropic etch of the floating gate layer.

CROSS REFERENCE TO RELATED APPLICATIONS

The present invention is a division of U.S. patent application Ser. No.10/798,475, filed Mar. 10, 2004, incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to integrated circuits, and moreparticularly to nonvolatile memories.

FIG. 1 illustrates an electrically erasable programmable read-onlymemory array (EEPROM) described in U.S. Pat. No. 6,420,231 issued Jul.16, 2002 to Harari et al. and incorporated herein by reference. FIG. 2is a circuit diagram of the array. Each memory cell 110 has twoconductive floating gates 120 positioned side by side in the X direction(row direction) over planar top surface 124T of silicon substrate 124.The floating gates are insulated from the substrate. In the Y direction(column direction), the adjacent floating gates are separated by fieldoxide regions 130. The floating gates are formed from the firstpolysilicon layer.

Steering gates 134 are formed from the second polysilicon layer and areinsulated from the floating gates. Each steering gate extends in the Ydirection between two columns of memory cells 110 and overlies twoadjacent columns of floating gates 120. Bitlines 138 are diffusionregions in substrate 124. Each bitline 138 runs in the Y directionbetween two adjacent columns of floating gates 120. In each row, abitline 138 provides two source/drain regions to respective two adjacentmemory cells 110.

Wordlines 144, formed from the third polysilicon layer, overlie thesteering gates and extend in the X direction. Wordlines 144 may also beformed from polycide. The wordline layer also provides select gates 144S(FIG. 1) for the memory cells.

Metal strap lines (not shown) reduce the resistance of the polysiliconelements and diffusion elements of the array.

The memory operates as follows. Each cell 110 can be represented ashaving two floating gate transistors 110L, 110R (FIG. 2) separated by aselect gate transistor 110S (a transistor with gate 144S). The floatinggate of transistor 110L is selected for reading or programming byplacing a sufficient voltage on the steering gate 134 above the floatinggate of transistor 110R to turn on the transistor 110R regardless of thecharge on its floating gate. Likewise, the floating gate of transistor110R is selected for reading or programming by placing a sufficientvoltage on the steering gate 134 above the floating gate of transistor110L to turn on the transistor 110L regardless of the charge on itsfloating gate. Each floating gate can be read by providing a voltagedifference between the respective bitlines 138 and sensing the state ofone of the bitlines. A negative charge can be written to a floating gateby source side hot electron injection. The floating gates can be erasedthrough wordlines 144 or substrate 124. See U.S. Pat. No. 6,266,278issued Jul. 24, 2001 to Harari et al. and incorporated herein byreference.

As noted above, floating gates 120 are made from the first polysiliconlayer, steering gates 134 are made from the second polysilicon layer,and wordlines 144 are made from the third polysilicon layer or apolycide layer which also provides the select gates 144S. Alternativefabrication techniques are desirable.

SUMMARY

This section summarizes some features of the invention. Other featuresare described in the subsequent sections. The invention is defined bythe appended claims which are incorporated into this section byreference.

In some embodiments of the present invention, select gates 144S areformed from a different layer than wordlines 144. In some embodiments,this fabrication method provides additional control over the spacingbetween the wordlines 144 and steering gates 134. Increased spacing isdesirable to reduce the parasitic capacitance between the wordlines andthe steering gates.

In some embodiments, select gates 144S are formed before floating gates120 and before steering gates 134 (the steering gates will also becalled “control gates” herein). Wordlines 144 are formed after thefloating and control gates. For example, select gates 144S can be formedfrom the first polysilicon layer, floating gates 120 from the secondpolysilicon layer, control gates 134 from the third polysilicon layer,and wordlines 144 from a metal layer. The gate dielectric for selectgates 144S can be formed by thermal oxidation before the formation ofthe floating gates. Therefore, the floating gates are not affected (notoxidized) by the select gate oxide growth. Also, in some embodiments,the select gate dielectric has the same thickness as the gate dielectricof high voltage peripheral transistors, so the same gate dielectriclayer can be used both for select gates 144S and the high voltageperipheral transistors. See U.S. patent application Ser. No. 10/440,508filed May 16, 2003 and Ser. No. 10/632,154 filed Jul. 30, 2003, bothincorporated herein by reference.

In addition, if the select gates 144S are formed before the floatinggates, the gate dielectric for floating gates 120 can be the same layeras the dielectric formed on the select gate sidewalls to insulate theselect gates from the floating and control gates. See U.S. patentapplication Ser. No. 10/440,005 filed May 16, 2003 and Ser. No.10/631,452 filed Jul. 30, 2003, both incorporated herein by reference.

In some embodiments of the present invention, the floating gates arefabricated in a self-aligned manner using an isotropic etch of thefloating gate layer.

Other features and advantages of the invention are described below. Theinvention is defined by the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a prior art memory array.

FIG. 2 is a circuit diagram of the array of FIG. 1.

FIGS. 3A, 3B are perspective views of a memory array according to oneembodiment of the present invention.

FIGS. 3C, 3D, 3E, 3F show vertical cross sections of the array of FIGS.3A, 3B.

FIG. 3G is a top view of the array of FIGS. 3A, 3B.

FIGS. 4–7, 8A, 8B, 9–12, 13A, 13B, 14A, 14B, 15A show vertical crosssections of memory structures in the process of fabrication according toone embodiment of the present invention.

FIG. 15B is a top view of a memory structure in the process offabrication according to one embodiment of the present invention.

FIGS. 15C, 16A, 16B, 16C show vertical cross sections of memorystructures in the process of fabrication according to one embodiment ofthe present invention.

FIG. 17A is a top view of a memory structure in the process offabrication according to one embodiment of the present invention.

FIGS. 17B, 17C, 18A, 18B, 18C, 19, 20A, 20B, 21, 22A, 22B, 23, 24A, 24B,25A, 25B, 26A, 26B, 26C, 27A, 27B, 28A, 28B show vertical cross sectionsof memory structures in the process of fabrication according to oneembodiment of the present invention.

DESCRIPTION OF SOME EMBODIMENTS

The embodiments described in this section illustrate but do not limitthe invention. The invention is not limited to particular materials,process steps, or dimensions. The invention is defined by the appendedclaims.

FIGS. 3A–3G are different views of a nonvolatile memory array accordingto one embodiment of the present invention. The array has the circuitdiagram of FIG. 2. FIG. 3A is a perspective view. FIG. 3B is the sameview but with an interlevel dielectric 310 (ILD). FIG. 3G is a top view.FIGS. 3C, 3D, 3E, 3F show vertical cross sections marked in FIG. 3G asX-C, Y-D, Y-E, and X-F respectively. The cross section X-C runs in the Xdirection (row direction) through floating gates 120 and the activeareas between field dielectric regions 130. The cross section Y-D runsin the Y (column) direction through select gates 144S. The cross sectionY-E runs in the Y direction through floating gates 120. The crosssection X-F runs in the X direction through oxide regions 130 betweenselect gates 144S.

As in FIG. 1, each memory cell has two conductive floating gates 120positioned side by side in the X direction over the planar top surfaceof active areas 312 of silicon substrate 124. The floating gates areinsulated from the substrate by dielectric 314. In the Y direction, theadjacent floating gates are separated by field oxide regions (substrateisolation regions) 130. In this embodiment, the substrate isolation isSTI type (shallow trench isolation). Oxide 130 is formed in trenches130TR. Each trench 130TR runs through the whole array, but oxide 130 isetched out of the trenches at the location of bit lines 138 (note FIG.3F showing a cross section along a trench 130TR). Oxide 130 protrudesupward above the top surface 124T of substrate 124.

The invention is not limited to STI. For example, the oxide 130structure of FIG. 1 can also be used.

In FIGS. 3A–3G, control gates 134 overlie floating gates 120, and areinsulated from the floating gates and from select gates 144S by ONO 324.ONO 324 (oxide/nitride/oxide) is a sandwich of silicon dioxide, siliconnitride, silicon dioxide. As in FIG. 1, each control gate 134 extends inthe Y direction between two columns of the memory cells and overlies tworespective adjacent columns of floating gates 120. Bitlines 138 arediffusion regions in substrate 124. Each bitline 138 runs in the Ydirection between two adjacent columns of floating gates 120, traversingthe trenches 130TR. In each row except the first and the last rows ofthe array, a bitline 138 provides two source/drain regions to twoadjacent memory cells.

In each memory cell, a channel region in the active area of substrate124 extends between the two adjacent bitlines. The channel regionincludes two channel areas underlying two respective floating gates 120and a channel area underlying the select gate 144S of the cell.

In this embodiment, control gates 134 are doped polysilicon silicidedwith cobalt silicide 134L.

Conductive select gates 144S are insulated from substrate 124 bydielectric 330. In this embodiment, select gates 144S are dopedpolysilicon silicided with cobalt silicide 144L. Each select gate 144Smay overlap the adjacent STI trenches 130TR (FIGS. 3D, 3G), but eachgate 144S does not extend to the adjacent memory cells. The wordlines144 (e.g. metal) are formed from a separate layer. Before that layer isdeposited, interlevel dielectric 310 is formed over the structure. Thiscan be a thick layer, as needed to reduce the capacitance between thewordlines 144 and the underlying memory features (including the floatingand control gates). In some embodiments, dielectric 310 is 200–300 Åthick, but its thickness can be 500 Å or more if needed. Contactopenings 340 are etched in ILD 310 to select gates 144S (i.e. to thesilicide features 144L). The openings are filled with metal 350. Metal350 may be part of wordline layer 144, or may be a separate layer (e.g.tungsten plugs).

The term “select gate” as used herein can be applied to the polysilicongate 144S or to a combination of polysilicon 144S and silicide 144L.Alternatively, the silicon/silicide combination may be called a “gatestructure” that includes a select gate 144S. Likewise, the term “controlgate” may refer to a polysilicon gate 134 or a combination of thepolysilicon gate and the overlying silicide 134L. Alternatively, thecombination may be called a “gate structure”. The term “control gate”may apply to a control gate of a single memory cell or to a line 134 (or134/134L) running through the array.

Exemplary operating voltages are shown in Table 1 below. The voltagesare in volts. The reading and programming operations are performed as inthe memory of FIG. 1. The programming is by channel hot electroninjection (CHIE). The erase is through substrate 124 (Fowler-Nordheimtunneling through dielectric 314). The sign “˜” indicates a voltagerange. For example “1˜2” means 1 V to 2 V.

TABLE 1 Program Read (CHEI) Erase WL 144 Selected 2.5 1.5 1~2 (Row)Unselected 0 0 0 CG 134 Selected Selected bit 1.5~2   9~10  −9~−10Column (L or R) Unselected bit 6~7 6~7 (R or L) Unselected Column 0 0 BL138 Selected Selected bit  1~1.5 4.5~5  Floating Column (L or R)Unselected bit 0 0 (R or L) Unselected Column 0 0 Substrate 124 0 0 7~8

The programming can also be done by Fowler-Nordheim tunneling.

In one embodiment, the memory is fabricated as follows. The memory arrayis fabricated in a doped region of type P- formed in monocrystallinesilicon substrate 124. This region can be isolated by P-N junctions (notshown). See e.g. U.S. Pat. No. 6,355,524 issued Mar. 12, 2002 to H. T.Tuan et al. and incorporated herein by reference.

STI trenches 130TR can be formed, for example, by a process described inU.S. patent application Ser. No. 10/678,317 US filed Oct. 3, 2003 by YiDing and incorporated herein by reference. More particularly, as shownin FIG. 4, silicon dioxide 410 (pad oxide) is formed on substrate 124 bythermal oxidation or some other technique to an exemplary thickness of150 Å. The dimensions, and the voltages in Table 1, are given for anexemplary process using a 0.18 μm line width technology (the line widthis the minimal dimension that can be reliably printedphotolithographically). The fabrication method is believed to bescalable to smaller line widths (e.g. 90 nm or even smaller), and theinvention is not limited to a particular line width. FIG. 4 shows a Ycross section, i.e. a cross section in the Y direction. All the Y crosssections are identical at this stage.

Silicon nitride 420 is deposited on oxide 410. In one embodiment, thethickness of nitride 420 is in the range of 2000 Å to 2200 Å. Nitride420 is patterned photolithographically, using a photoresist mask (notshown), to define trenches 130TR and active areas 312. Oxide 410 andsubstrate 124 are etched through the openings in nitride 420. Trenches130TR (“STI trenches”) are formed as a result. An exemplary depth oftrenches 130TR is 0.2 μm, measured from the top surface of the substrate124. Other depths are possible.

Nitride 420 is subjected to a wet etch to recess the vertical edges ofthe nitride layer away from trenches 130TR. See FIG. 5 (Y crosssection). This step reduces the aspect ratio of the holes that will befilled with dielectric 130. As will be seen below, the etch of nitride420 will also reduce the capacitance between the floating gates and thesubstrate 124, thus increasing the gate coupling ratio.

A thin layer 130.1 of silicon dioxide is thermally grown on the exposedsilicon surfaces to round the edges of trenches 130TR and passivate thetrench surfaces. Silicon dioxide 130.2 (FIG. 6, cross section Y) isdeposited by a high density plasma process (HDP) or by non-plasma lowpressure chemical vapor deposition (LPCVD). Oxide 130.2 fills thetrenches and initially covers the nitride 420. Oxide 130.2 is polishedby CMP (chemical mechanical polishing). The CMP stops on nitride 420. Aplanar top surface is provided.

In some of the figures, the layers 130.1, 130.2 are shown as a singlelayer 130. This dielectric 130 will be referred to as STI dielectric orfield dielectric.

The array area is masked with photoresist (not shown), and oxide 130 isetched to lower the oxide level in the periphery. See FIG. 7 showing theY cross section of the array and a peripheral cross section. The loweroxide level will increase the peripheral surface planarity, thusfacilitating the subsequent fabrication steps. In some embodiments, thetop level of oxide 130 is lowered to a level of 200–500 Å above thesubstrate 124 in the periphery.

The photoresist is removed. Nitride 420 is removed selectively to oxide130. This can be done by a wet etch (e.g. with phosphoric acid). SeeFIG. 8A (cross section Y) and FIG. 8B (periphery). Then pad oxide 410(FIG. 7) is etched away to expose the substrate 124 in the active areas.The oxide etch may remove a small amount of oxide 130.

Silicon dioxide 330 is thermally grown on the exposed areas of substrate124 to provide gate dielectric for the select gates of the memory arrayand for the peripheral transistors. The peripheral transistors are usedto form sense amplifiers, address decoders, memory I/O buffers, driversfor various memory elements (e.g. bitlines, wordlines, control gates),and possibly other circuitry needed to access the memory array. See e.g.the aforementioned U.S. Pat. Nos. 6,420,234 and 6,266,278. An exemplarythickness of oxide 330 in the array area is 120 Å. Generally, the oxidethickness depends on the maximum voltage that the oxide 330 is designedto sustain during the memory operation.

In the example shown in FIG. 8B, the peripheral area includes a highvoltage transistor area 812H and a low voltage transistor area 812L.Oxide 330 is first grown thermally to a thickness of 60 Å over theentire wafer. This oxide is removed from the low voltage area 812L by amasked etch. The wafer is re-oxidized to re-grow silicon dioxide 330 inarea 812L to a thickness of 60 Å. The oxide thickness in the memoryarray area and in high voltage area 812H increases from 60 Å to 120 Åduring this step.

Thus, oxide 330 in the array area and oxide 330 in the high voltageperipheral area 812H is formed simultaneously in these two oxidationsteps. The oxide 330 in area 812L and the oxide 330 in the array areaand area 812H are not formed simultaneously because the oxide 330 inarea 812L is formed in the second oxidation step. See U.S. patentapplication Ser. No. 10/440,508 filed May 16, 2003 by Yi Ding andincorporated herein by reference.

Intrinsic polysilicon 144.1 (FIG. 9, cross section Y and peripheralcross section) is deposited over the wafer. Layer 144.1 will provideportions of select gates 144S and peripheral transistor gates. Anexemplary deposition process is LPCVD, and an exemplary thickness is1000–1400 Å. Polysilicon 144.1 fills the recesses between the STIdielectric regions 130 and covers the whole wafer.

Silicon dioxide 902 is deposited on polysilicon 144.1 to an exemplarythickness of 1200–1500 Å by CVD (TEOS) or some other process.

A photoresist mask 904 is formed over the periphery. Oxide 902 is etchedaway in the array area, and polysilicon 144.1 is doped N+ by ionimplantation. See FIG. 10 (cross section Y and peripheral crosssection). Alternatively, the doping can be performed before the etch ofoxide 902, by ion implantation through the oxide. Resist 904 blocks thedopant from the periphery. Polysilicon 144.1 remains undoped in theperiphery, and will be doped later together with the peripheralsource/drain regions to create surface channel peripheral transistors.These details are exemplary and not limiting. Non-surface-channeltransistors can also be used in the periphery, and further the surfacechannels transistors can be created by other techniques, known or to beinvented.

Polysilicon 144.1 is subjected to a timed dry anisotropic etch to lowerits top surface below the top surface of dielectric 130 in the arrayarea. See FIG. 11 (cross section Y and peripheral cross section). Resist904 protects the periphery during this etch.

Oxide 130 is etched selectively to polysilicon 144.1 (FIG. 12, crosssection Y). The etch includes a lateral component that causes thesidewalls of oxide 130 to be laterally recessed in the direction awayfrom the adjacent polysilicon features 144.1 and active areas 312. Thiscan be an isotropic wet etch. In one embodiment, the isotropic etchlaterally recesses the sidewall by an amount Ls in the range of 0.04 μmto 0.05 μm. The etch also lowers the top surface of oxide 130 by thesame amount. In addition, the etch attacks the oxide portions near thepolysilicon 144.1 to form pockets 1210 in which the top surface of oxide130 is below the top surface of polysilicon 144.1. Between the pockets1210, the top surface of oxide 130 is shown to be above the top surfaceof polysilicon 144.1, but this is not necessary. The top surface ofoxide 130 between the pockets 1210 may be even with, or below, the topsurface of polysilicon 410.1. See U.S. patent application Ser. No.10/678,317 filed Oct. 3, 2003 by Yi Ding and incorporated herein byreference.

The periphery is protected by resist 904, and remains as in FIG. 11.

As seen in FIG. 3E, the recessed sidewalls of oxide 130 will allow thetop surface of floating gates 120 to extend over the oxide 130, possiblybeyond the active areas 312, advantageously increasing the capacitivecoupling between the floating and control gates.

Resist 904 is removed. Polysilicon layer 144.2 (FIG. 13A, cross sectionY, and FIG. 13B, periphery) is deposited on the structure, and is dopedN+ during or after the deposition. Layer 144.2 will provide portions ofthe select gates 144S. An exemplary deposition process is conformal lowpressure chemical vapor deposition (LPCVD). The thickness of layer 144.2(at least 2000 Å in some embodiments) is chosen to provide a planar topsurface. The planar top surface is not necessary however.

Polysilicon layers 144.1, 144.2 are shown as a single layer 144S in thearray area in some of the drawings.

A hard mask for an etch of polysilicon layers 144.1, 144.2 is formed intwo steps. First, silicon nitride layer 1310 is deposited and patternedphotolithographically to form the same pattern in the array area as wasused to define active areas 312 (FIG. 4). Nitride 1310 covers the entireperiphery. Next, a conformal silicon nitride layer 1410 (FIG. 14A, crosssection Y) is deposited and etched anisotropically without a mask toform spacers on the sidewalls of nitride 1310. Nitride 1410 is etchedoff the periphery during this step, so the periphery remains as in FIG.13B. Nitride spacers 1410 will ensure that the select gates 144S willoverlap the top planar surface of STI oxide 130 even if nitride 1310 isnot perfectly aligned with active areas 312.

Alternatively, nitride 1310 can be removed from the periphery when thenitride 1310 is patterned in the array area. Nitride 1410 will also beremoved from the periphery during the etch that forms the nitridespacers in the array area. The resulting peripheral cross section forthis case is shown in FIG. 14B.

An anisotropic etch of polysilicon 144S (FIG. 15A, cross section Y, andFIG. 15B, top view of the array) stops on STI oxide 130. Some of oxide130 may be removed by an overetch. Polysilicon 144S forms a number ofstrips (FIG. 15B); each strip runs through the memory array in the Xdirection over an active area 312. (Nitride layers 1310, 1410 are notshown in FIG. 15B.)

If the periphery was as in FIG. 13B, it will not be affected by thepolysilicon etch. If the periphery was as in FIG. 14B, polysilicon 144.2will be removed from the periphery, as shown in FIG. 15C.

Advantageously, the array structure is essentially self-aligned at thisstage. The positions of trenches 130TR, oxide 130 and polysilicon 144Sdoes not depend on photolithographic alignment except for the alignmentof the mask used to pattern the nitride 1310 with respect to the maskused to pattern the trenches 130TR (FIG. 4). A misalignment betweenthese two masking steps may cause a shift of polysilicon strips 144Srelative to oxide 130 in the Y direction, but the shift is not believedto affect the memory characteristics if nitride spacers 1410 aresufficiently wide to ensure the overlap of the polysilicon strips ontothe top surface of oxide 130 (as in FIG. 15A).

Nitride layers 1310, 1410 are removed from the wafer (by a wet H₃PO₄etch for example). Silicon nitride 1610 (FIG. 16A, cross section Y) isdeposited over the structure to provide a planar top surface (the planartop surface is not necessary however). The peripheral cross section isshown in FIGS. 16B, 16C for the cases of FIGS. 13B, 15C respectively. Anexemplary thickness of nitride 1610 over the top surface of polysilicon144S is in the range of 500–1500 Å. A SION or silicon dioxide layer 1620is formed on nitride 1610 to provide additional protection for asubsequent polysilicon etch that will define the select gates 144S.Layer 1620 is not needed if nitride 1610 is sufficiently thick (1500 Åfor example).

Layers 1610, 1620 are patterned photolithographically using aphotoresist layer (not shown) to form a number of strips running throughthe array in the Y direction over the future positions of select gates144S. See FIG. 17A (top view of the array). The photolithographic stepis not alignment-sensitive in the array area because any misalignmentwill simply shift the strips 1610, 1620 in the X direction and will notaffect the array geometry. Layers 1610, 1620 are removed from theperiphery during this step, as shown in FIG. 17B (for the case of FIG.16B) and FIG. 17C (for the case of FIG. 16C).

The resist used to pattern the layers 1610, 1620 can be removed at thisstage, or can be left in the structure for the etch of polysilicon 144S.Polysilicon 144S is etched anisotropically to form the select gates144S. The etch is selective to SION 1620 and nitride 1610, or at leastto the resist if the resist is present. See FIG. 18A showing the crosssection X-C (marked in FIG. 3G), and FIG. 18B showing the cross sectionX-F. A dashed line in FIG. 18A shows the position of the top surface130T of STI oxide 130 between select gates 144S (see also FIG. 16A). Inthe case of FIG. 17B, polysilicon 144.2 is removed from the peripheryduring this etch, so the periphery becomes as in FIG. 17C. See also FIG.18C, showing the periphery both for the case of FIG. 17B and the case ofFIG. 17C after the polysilicon etch.

If the resist was present during the polysilicon etch, the resist isremoved. The wafer is oxidized (in a furnace or by rapid thermaloxidation (RTO)) to grow silicon dioxide 314 (FIG. 18A) on substrate124. During this step, silicon dioxide 1820 grows on the sidewalls ofpolysilicon gates 144S in the array area, and a thin oxide layer 1830grows on the vertical sidewalls of nitride 1610. Oxide 1830 may alsoform on the top surface of nitride 1610 if SION 1620 is omitted. Oxide314 will serve as the tunnel oxide (the gate oxide for the floating gatetransistors). Oxide 1820 will provide sidewall insulation for the selectgates. The oxide thickness depends on the dopants and dopantconcentrations, and is chosen based on a desired memory cellperformance. In one embodiment, oxide 314 is 60–100 Å thick, and oxide1820 is 250–450 Å thick (due to the heavier doping of select gates144S).

The peripheral area is covered by oxide 902 (FIG. 18C), and is notaffected by the oxidation.

Floating gate polysilicon 120 (FIG. 19, cross section X-C) is depositedover the structure, by LPCVD for example, and is doped during or afterthe deposition. Polysilicon 120 is sufficiently thick to ensure that itstop surface is at least as high throughout the wafer as the surface 130T(FIG. 16A) of STI oxide 130. In the embodiment of FIG. 19, thepolysilicon surface is planar except for protrusions over the selectgates 144S. The surface 120T of polysilicon 120 between the STI regions130 is planar due to a conformal polysilicon deposition if the maximumdistance D1 (FIGS. 3B, 3E) between adjacent STI oxide regions 130 abovesubstrate 124 is not larger than twice the thickness Th1 (FIG. 19) ofpolysilicon 120, i.e. D1≦2*Th1. The polysilicon protrusions over selectgates 144S (FIG. 19) are obtained if twice the thickness Th1 ofpolysilicon 120 is smaller than the distance D2 between the adjacentstructures consisting of select gates 144S and oxide 1820 (see also FIG.18A). In summary,D1≦2*Th1<D2.

In some 0.18 μm minimum line width embodiments, D1 is 0.16–0.2 μ, D2 is0.58 μm, and Th1 is at least 0.08 μm but less than 0.29 μm.

In other embodiments, the polysilicon surface is planar over the wholewafer due to a conformal deposition to a thickness larger than half thedistance D2 and larger than half the distance D1. See U.S. patentapplication Ser. No. 10/440,508 filed May 16, 2003 by Yi Ding andincorporated herein by reference.

In other embodiments, the polysilicon 120 is not planar even in theareas between select gates 144S. For example, polysilicon 120 can haveprotrusions over STI oxide regions 130.

Polysilicon 120 is etched without a mask. In the embodiment of FIG. 19,this can be an isotropic dry etch. The etch end point is the exposure ofoxide surface 130T. See FIGS. 20A (cross section X-C) and 20B (crosssection Y-E). The etch removes polysilicon 120 over the select gates144S, and also removes SION 1620. The remaining polysilicon 144S fillsthe areas between protruding oxide features 130 and select gates 144S.Oxide layers 1820, 1830 become exposed. Oxide 1830 protects the nitride1610 during the etch, and may be partially or completely removed by thisetch. The polysilicon etch removes polysilicon 120 from the periphery,so the periphery becomes as in FIG. 18C.

Anisotropic etches can also be used, especially if polysilicon 120 isinitially planar throughout the wafer. If the polysilicon is not planar,it can be planarized before the anisotropic etch.

An optional etch of oxide 130 lowers the top surface of oxide 130 to alevel below the top surface of polysilicon 120 (FIG. 21, cross sectionY-E) to increase the capacitive coupling between floating gates 120 andcontrol gates 134 (FIG. 3E). See the aforementioned U.S. Pat. No.6,355,524. The oxide etch can be an anisotropic dry etch. Some of oxide902 can be removed in the periphery by this etch (see FIG. 18C).

As noted above in connection with FIG. 12, the recessed sidewalls ofoxide 130 allow the floating gates to be wider at the top (by the amountLs) and to possibly extend beyond the active areas 312. See also FIGS.3A, 3B, 3E. The lateral extensions of the floating gates increase thecapacitive coupling between the floating gates and the control gates 134and increase the gate coupling ratio. Also, due to the lateralextensions, the spacing between the floating gates is reduced, possiblybelow the minimum line width, thus allowing a more efficient use of thewafer area. In one embodiment, the spacing Sf (FIGS. 3E, 21) between theadjacent floating gates is only 0.05–0.06 μm for the dimensions givenabove in connection with FIG. 12.

The wafer is cleaned for a silicon nitride deposition. The cleaning stepremoves any oxide 1830 (FIGS. 18A, 18B, 20A) that may have remained inthe structure after the isotropic etch of polysilicon 120. A conformalsilicon nitride layer 2210 (FIG. 22A, cross section X-C, and FIG. 22B,cross section X-F) is deposited and etched anisotropically to formspacers on the sidewalls of select gates 144S and nitride 1610. In oneembodiment, the spacer width Ws is 0.18–0.2 μm at the bottom overpolysilicon 120. This parameter will control the width of the floatinggates.

Nitride 2210 is etched off the periphery during this etch, so theperiphery remains as in FIG. 18C.

Polysilicon 120 is etched anisotropically selectively to silicon nitrideto create the floating gates (FIG. 23, cross section X-C). The etch isselective to silicon dioxide, so the periphery remains as in FIG. 18C.

Then bitlines 138 are created by a “self-aligned source” technique. Moreparticularly, the peripheral area is masked with photoresist (notshown), and a silicon dioxide etch selective to silicon nitride removesthe exposed oxide 130 from trenches 130TR. See FIG. 24A (cross sectionX-F) and FIG. 3F. Substrate 124 becomes exposed in the bitline areas 138(FIG. 3G). Dopant is implanted into these areas to form the bitlines.See FIG. 24A, FIG. 24B (cross section X-C), and FIG. 3G. In an exemplaryembodiment, a combination of a shallow arsenic implant and a deepphosphorus implant are used for the bitlines.

The resist is removed from over the periphery. The periphery remains asin FIG. 18C.

The structure is oxidized to grow a silicon dioxide layer 2410 on thesidewalls of floating gates 120 and the exposed portions of substrate124. This step repairs the damage to oxide 314 caused by the etch ofpolysilicon 120 that formed the floating gates (see FIG. 23). Oxide 2410is a thin layer whose thickness depends on the doping of the underlyingsilicon surfaces. The periphery remains as in FIG. 18C.

A wet silicon nitride etch (e.g. with H₃PO₄) removes the nitride spacers2210. This etch also attacks the nitride 1610. Nitride 1610 may becompletely removed from over the select gates 144S, but in someembodiments the nitride 1610 is not completely removed between theselect gates (see FIGS. 3D, 3F) due to a large nitride thickness inthese areas.

The periphery is covered by oxide 902, and remains as in FIG. 18C.

Insulating layer 324 (FIG. 25A, cross section X-C, and FIG. 25B,periphery), e.g. ONO, is formed over the structure. A conductive layer134, e.g. doped polysilicon, is deposited over ONO 324. In theembodiment shown, layer 134 is deposited conformally and has a planartop surface, although this is not necessary. An etch of polysilicon 134removes the polysilicon from over the select gates 144S (FIG. 26A, crosssection X-C) and the top surface of nitride 1610 (FIG. 26B, crosssection X-F, and FIG. 26C, periphery). The etch could stop when ONO 324becomes exposed, but in the embodiment shown in the figures the etchcontinues so as to lower the top surface of polysilicon 134 in the arrayarea to a level below the top surface of select gates 144S. The etchstops when polysilicon 134 still covers the floating gates 120, toprovide a large capacitive coupling between the floating and controlgates. In some embodiments, the final thickness of polysilicon 134 is500 Å above the floating gates.

The etch of polysilicon 134 completely clears the polysilicon from theperiphery.

A silicon dioxide etch (e.g. a wet etch selective to silicon nitride)removes the top oxide sub-layer of ONO 324 in the periphery (FIG. 27A).This etch also removes the top oxide layer over the select gates 144S(FIG. 27B, cross section X-C). The remaining nitride and oxidesub-layers of ONO 324 are shown as 324.1 in FIG. 27B. Layer 324.1 is notmarked separately in the remaining figures, but is simply shown as layer324 (see FIG. 3C for example). The oxide etch can be omitted if theoxide was removed during the etch of polysilicon 134 (FIGS. 25A, 25B,26A–26C). Then the array is covered with photoresist (not shown). Theremaining (nitride and bottom oxide) portions of ONO 324 and the oxide902 are removed in the periphery (possibly with a wet etch). The resistis removed, and another photoresist layer (not shown) is formed todefine the peripheral transistor gates. Polysilicon 144.1 is patternedin the periphery to form these transistor gates. The resist is removed.Then LDD (lightly doped drain) extensions for peripheral source/drainregions 2708 are formed by ion implantation for the PMOS and NMOStransistors using appropriate photoresist masks (not shown). Theperipheral PMOS gates 144.1 are doped P+, and the peripheral NMOS gatesare doped N+, simultaneously with the LDD extensions. Surface channeltransistors are formed in the periphery as a result. A thin pad layer2703 of silicon dioxide is grown on the top and sidewall surfaces ofperipheral gates 144.1 and on control gates 134 using known techniques.

Silicon nitride spacers 2710 are formed by a conformal deposition and ananisotropic etch of silicon nitride. This etch also removes the nitridesub-layer of layer 324.1 in the array area. Then N+ and P+ implants areperformed to finish the doping of source/drain regions 2708 for theperipheral transistors. Appropriate masking steps are used to blockthese implants from the wafer areas in which the doping is undesirable.Then a wet silicon dioxide etch is performed to remove the remainingoxide sub-layer of layer 324.1 and also to remove the exposed portionsof oxide 2703 over the control gates 134 and the peripheral transistorgates 144.1. See FIG. 28A (periphery) and FIG. 28B (cross section X-C).This etch also removes the exposed portions of oxide 330 in theperiphery over the silicon source/drain regions 2708. A metal (e.g.cobalt) is deposited over the wafer, and the wafer is heated to formconductive metal silicides 134L, 144L, 2708L on respective siliconregions 134, 144, 2708. The unreacted metal is removed.

The remaining processing is conventional. In one example, a thin siliconnitride layer 2950 (FIGS. 3B, 3C, 3D, 3F) is deposited over thestructure to protect the peripheral source/drain regions 2708 during acontact etch of interlevel dielectric 310. Dielectric 310 is formed overthe structure, to an exemplary thickness of 200–300 Å, or possibly 500 Åor more. Contact openings are etched in dielectric 310 and nitride 2950to select gates 144S, control gates 134, and peripheral transistor gates144.1 and source/drain regions 2708 (to respective silicide features144L, 134L, 2708L). The etch of dielectric 310 stops on nitride 2950,and a short etch of nitride 2950 is performed to finish the contactopenings. The contact openings are filled with metal 350, then metal 144is deposited and patterned to form the wordlines and perhaps otherfeatures.

The invention is not limited to the embodiments described above. Theinvention is not limited to the dimensions, materials or voltages shown,or to STI, silicidation, or other processes. In FIGS. 3A–3G, wordlines144 are perpendicular to control gate lines 134 and bitlines 138, butthis is not necessary. The invention is applicable to non-siliconsemiconductor memories. The invention is not limited to a memory erasedthrough the substrate, or to any reading, erase or programming methods.The invention covers both flash and non-flash memories. Otherembodiments and variations are within the scope of the invention, asdefined by the appended claims.

1. An integrated circuit comprising: a plurality of first conductivegate structures, each first conductive gate structure comprising a firstconductive gate of a nonvolatile memory cell, the first conductive gatestructures being spaced from each other, the first conductive gatescomprising a semiconductor material; a plurality of conductive floatinggates for the memory cells; a plurality of second conductive gates forthe memory cells; at least one metal line electrically interconnectingtwo or more of the first conductive gates.
 2. The integrated circuit ofclaim 1 further comprising a dielectric overlying the floating gates andthe second conductive gates and underlying the metal line.
 3. Theintegrated circuit of claim 2 further comprising a semiconductorsubstrate, wherein the first and second conductive gates are formed overthe semiconductor substrate above a top of the semiconductor substrate;wherein the dielectric is at least 200 Å thick.
 4. The integratedcircuit of claim 2 wherein the dielectric is at least 500 Å thick. 5.The integrated circuit of claim 1 comprising at least one conductiveline which provides the second conductive gates to at least two adjacentcolumns of the memory cells.
 6. The integrated circuit of claim 5wherein each metal line electrically interconnects the first conductivegates for at least one row of the memory cells.
 7. The integratedcircuit of claim 5 wherein each memory cell comprises two of thefloating gates.
 8. The integrated circuit of claim 1 comprising asemiconductor substrate, wherein each memory cell comprises a channelarea underlying a floating gate and a channel area underlying a firstconductive gate.
 9. The integrated circuit of claim 8 wherein eachmemory cell comprises two floating gates and two channel areasunderlying the two floating gates.
 10. The integrated circuit of claim 1wherein a plurality of the second conductive gates are interconnected bya conductive line perpendicular to the metal line.
 11. The integratedcircuit of claim 1 further comprising a semiconductor substrate, whereinthe first conductive gates are formed over the semiconductor substrateabove a top of the semiconductor substrate.
 12. The integrated circuitof claim 1 further comprising one or more substrate isolation regions ina semiconductor substrate between active areas of the semiconductorsubstrate; wherein at least one of the first conductive gates overlapsat least one of the substrate isolation regions.
 13. The integratedcircuit of claim 12 wherein at least one of the first conductive gatesoverlaps two of the substrate isolation regions.
 14. The integratedcircuit of claim 1 wherein in each memory cell, each second conductivegate is present over and on a side of a corresponding one of thefloating gates.
 15. An integrated circuit comprising: a plurality offirst conductive gate structures, each first conductive gate structurecomprising a first conductive gate of a nonvolatile memory cell, thefirst conductive gate structures being spaced from each other; aplurality of conductive floating gates for the memory cells; a pluralityof second conductive gates for the memory cells; a dielectric overlyingthe floating gates and the second conductive gates; at least oneconductive line overlying the dielectric and electricallyinterconnecting two or more of the first conductive gates; wherein thedielectric is at least 500 Å thick.
 16. The integrated circuit of claim15 comprising at least one conductive line which provides the secondconductive gates to at least two adjacent columns of the memory cells.17. The integrated circuit of claim 16 wherein each memory cellcomprises two of the floating gates.
 18. The integrated circuit of claim15 comprising a semiconductor substrate, wherein each memory cellcomprises a channel area underlying a floating gate and a channel areaunderlying a first conductive gate.
 19. The integrated circuit of claim18 wherein each memory cell comprises two floating gates and two channelareas underlying the two floating gates.
 20. The integrated circuit ofclaim 15 wherein a plurality of the second conductive gates areinterconnected by a conductive line perpendicular to the conductive linecontacting the first conductive gate structures.
 21. The integratedcircuit of claim 15 further comprising a semiconductor substrate,wherein the first conductive gates are formed over the semiconductorsubstrate above a top of the semiconductor substrate.
 22. The integratedcircuit of claim 15 further comprising one or more substrate isolationregions in a semiconductor substrate between active areas of thesemiconductor substrate; wherein at least one of the first conductivegates overlaps at least one of the substrate isolation regions.
 23. Theintegrated circuit of claim 22 wherein at least one of the firstconductive gates overlaps two of the substrate isolation regions. 24.The integrated circuit of claim 15 wherein in each memory cell, eachsecond conductive gate is present over and on a side of a correspondingone of the floating gates.